OverviewHermine came ashore shortly after midnight in the Florida Panhandle region as a category 1 hurricane and this ended the unprecedented hurricane drought for the state. Florida had not seen a hurricane of any intensity since Wilma struck the southwestern part of the state as a major (category 3) hurricane in October 2005. Hermine is now classified as a “tropical storm” and is located over eastern Georgia with max sustained winds at 50 mph and movement to the NE at 18 mph. Hermine will move northeast over the next 24 hours and then push off the Mid-Atlantic coastline later tomorrow. Once off the coast, Hermine is likely to regain strength - likely back to hurricane status - turn north and gradually slow down to a grinding halt. As a result, Hermine is likely to have a major impact on the coastline from Long Island to SE Virginia in what will turn out to be a long duration event of high winds and heavy rain. Rain and wind is also possible during this event back to the I-95 corridor region from DC-to-Philly-to-NYC.

Forecast DiscussionThe latest satellite image of Hermine is quite impressive with the familiar spiral pattern of a tropical storm. The cloud bands extending southward on the right side of the storm reach the Gulf of Mexico and Caribbean Sea suggesting copious amounts of moisture is feeding into the system. Hermine will generally move to the northeast over the next 24 hours along or near the Carolina coastline and then push off the Mid-Atlantic coast by later tomorrow. Once off the coast and out over the water, Hermine will become intertwined with a frontal boundary zone and increasingly under the influence of building high pressure across the Northeast US and southeastern Canada. As a result, Hermine’s movement will shift to more of a northerly direction than northeast, and its forward progress will slowly grind to a halt as a blocking pattern forms in the upper atmosphere. It is this deceleration that will lead to a long duration event for the Mid-Atlantic coastline from Long Island to SE Virginia later this weekend and into the early/middle parts of next week.

Another big concern is that the tropical system could actually regain some strength once out over the very warm waters off the Mid-Atlantic coastline. In some areas, the water temperature is currently 26°C or higher and this level has been found empirically to be a threshold allowing for the intensification of tropical cyclones. It is not out of the question that Hermine regains enough strength to reach hurricane status once again while off the Mid-Atlantic coastline. [Note- it is possible that the NOAA/National Hurricane Center will classify it as “extra tropical” as it interacts with a frontal zone; however, the impact on the Mid-Atlantic coastline will remain the same no matter how it is officially classified]. Hurricane-force winds are likely to be reached just offshore during this event - if not actually right on the coastline.

Current sea surface temperatures; map courtesy Weather Bell Analytics

This will be a situation in which there will be considerably more rain and wind closer to and at the coast and less inland; however, significant rain and wind can back into the immediate I-95 corridor (e.g., Philly, DC, NYC) at any time from late Saturday into the early part of next week. Coastal flooding, dangerous rip currents and beach erosion are serious concerns given the expected slow-down and potential re-intensification of this system which will allow for a long period of northeast winds - and water to pile up. Some model forecasts suggest wave height surpasses 30 feet during this storm just off the Mid-Atlantic coastline and this, of course, raises a red flag. A forecast for a specific buoy just south of Cape May, NJ suggests swells will surpass 26 feet by Labor Day (Monday).

Computer model forecastsComputer model forecasts continue to suggest Hermine will grind to a half off the Mid-Atlantic coastline from Long Island to SE Virginia and it could persist as a problem for an extended period of time from later this weekend into the middle part of next week. The 12Z NAM features a powerful storm off the Mid-Atlantic coastline on Sunday night with rain all the way to the I-95 corridor and the storm moves very little by 24 hours later (Monday night). The 00Z Euro also suggests Hermine grinds to a halt as its forecast map for Sunday night does not change much at all in the placement of the storm by 48 hours later (Tuesday night).

Some similarities to Hurricane AgnesIt is always useful to go back in time to try to find analog cases in which current conditions mimic closely prior events to get a better idea as to what could happen. One interesting and close analogy in terms of the potential track and intensification prospects of Hermine is Hurricane Agnes which produced historic flooding across much of Pennsylvania in June 1972. Of course, there are some differences between the two storms such as water temperatures which are higher now as compared with June 1972 for Agnes and will not be the flooding in Pennsylvania like there was back then, but the track and intensity changes may be quite similar.

Agnes came ashore in the Florida Panhandle then moved northeast to a position off the Mid-Atlantic coastline, regained strength from a tropical depression to a tropical storm, and slowed down considerably once off the Mid-Atlantic coastline. In fact, Agnes ended up looping back inland before stalling out and dissipating over northern Pennsylvania. Similar to Agnes, Hermine also can regain strength after it moves back over water and slows down in its forward progress. As to whether Hermine will actually loop back inland remains an open question, but it is certainly not off the table.

All eyes in the Mid-Atlantic region should continue to closely monitor this system over the next few days; especially, along the coastline as a small shift can make a huge difference.